so in trucks made now, and sold in the USA, there is a requirement that they have an operator in place or lift/lower and drive are disrupted. this makes a seat switch something that is even on Internal Combustion units. this way you still have motor operation to steer, brake, (etc...), but must be in the seat to lift lower or drive. many also have alarms and or lights for the seatbelt. this makes it almost trivial to wire the seat switch and seatbelt switch in series (instead of the parallel circuits they normally are) thus requiring operator in seat, and seatbelt on before lifting lowering or travel.
The only way I know to prevent an operator from connecting the seatbelt behind him is to explain that OSHA's fine for a routine disabling of a safety device (as opposed to the normal mistaken failure to use a required safety device) can be a multiple of the normal fine, as much as 10 times the fine for failure to enforce the routine use of a safety device. and there can be no arguement that a seatbelt attached but then sat upon is a "routine disabling" of a safety device